This invention relates to economical, accurate, and high-speed means for the conversion of analog signals to digital codes, more particularly, to codes with the unit-distance property, also known as Gray codes or cyclic binary codes. The process of converting analog electrical signals to digital codes is indispensible for the digital processing of information represented by these signals. With the enormous decrease in cost of digital processing in recent years with the appearance of integrated microprocessors, such processing is becoming universal. It appears in measuring instruments, process control, planned systems for automobile fuel economy and pollution reduction, in radar systems, in data transmission, exchange of television programs between countries with divergent standards, the processing of images in earth resources inventory from satellite photographs and medical imagery of different sorts, and other branches of technology, science, and industry.
A great diversity of analog/digital converters intended for these applications is known, and the state of the art is well described, for example, in the proceedings of the Symposium on Analog/Digital Conversion, held at the Swiss Federal Polytechnic Institute of Lausanne in 1973, "Comptes Rendus des Journees d'Electronique 1973 sur le theme Conversion A/D & D/A", EPFL, Lausanne, October, 1973.
To obtain fast and accurate conversion, such as is necessary for radar, television, and similar applications, the only hitherto known means is the so-called "parallel converter", in which the signal to be converted is compared to a multiplicity of reference voltages, each voltage representing one of the possible discrete values of the encoded output signal. Thus, if the signal is to be encoded by n bits, 2.sup.n -1 comparators must be used. Because comparators must transfer their outputs from one level, "high", to a second level, "low", for a very small change in input signal, they are required to have a very high "slew rate", which implies a high power consumption. An example of such a converter is shown on page 80 of the cited publication. Ideally, if there is no significant error in the comparators, all of the comparators whose thresholds are lower than the instantaneous value of the signal will have their outputs at one level, say low (or logical 0), and all of the comparators whose thresholds are above the instantaneous value of the signal will have their outputs at the other level, say high (or logical 1), with the possibility that one comparator, the comparator at the frontier between these two conditions, will have an indeterminate value. Thus, the signal to be converted is represented at the outputs of the comparators by a code in 2.sup.n -1 bits. Following the bank of comparators, therefore, is an encoder, that transforms the code in 2.sup.n -1 bits to a code in n bits. To avoid difficulties due to signal fluctuation during this encoding process, which is not instantaneous, such encoding is conveniently done in a binary reflected or unit-distance code, also called Gray code, after its inventor, and described in U.S. Pat. No. 2,632,058 of Mar. 17, 1953. This code has the property that only one bit changes at a time in going from one encoded value to another, so that if the input signal varies during the encoding time, the resulting uncertainty affects only one bit, provided that the signal fluctuation has a magnitude that passes from one level to an adjacent one only.
The properties of the Gray code have been exploited in a cascade converter, in which only one comparator per bit of output code is required, as described in the article by B. D. Smith, "An unusual electronic analog-digital conversion method", IRE Trans. on Instrum., PGI-5, June, 1956, pp. 155-160. However, in high-precision converters, involving 8 or more bits, the accumulation of static and dynamic errors in the cascade converters make them difficult of realization.
An attempt is also known to provide an analog-digital conversion process which combines the advantages in high-speed and accuracy of the parallel conversion with the property of the cascade-Gray conversion that only one comparison per bit is required.
In Federal German Pat. No. 1,268,196, filed July 26, 1966, Jungmeister describes a parallel Gray-code analog/digital converter in which the properties required of the various channels, one channel per bit, are obtained by a method of compensation of two highly non-linear transfer characteristics, actuating a bistable device with hysteresis nonlinearity, in the embodiment given there a tunnel diode. The nonlinear characteristics are obtained by series-parallel combinations of resistors and ordinary diodes, and are specific to each channel. The disadvantages of such an approach are two-fold. As is well known, the scatter of the switching characteristics of tunnel diodes is such that unless each channel is individually aligned, which precludes large-scale production by automatic techniques, the uncertainties of switching level render the device unusable for high accuracies and, secondly, the variations in voltage drop across the ordinary diodes used to obtain the nonlinear characteristics will have the same result.